Carboxyl

Carboxyl is a functional group consisting of a carbon atom double-bonded to an oxygen atom (C=O) and also bonded to a hydroxyl group (OH). This group is denoted as -COOH. Carboxyl groups are commonly found in organic chemistry and are characteristic of the organic compounds known as carboxylic acids.

Structure and Properties

The carboxyl group combines the properties of the carbonyl and hydroxyl groups. The double bond between the carbon and oxygen is highly polar, making the carboxyl group a site of high electron density, which can attract electrophiles. The presence of the hydroxyl group allows for hydrogen bonding, making carboxylic acids more soluble in water than comparable hydrocarbons.

Carboxylic acids can donate a hydrogen ion (proton) due to the polarity of the O-H bond within the carboxyl group, making them acidic. The acidity of carboxylic acids is higher than alcohols and phenols, largely due to the resonance stabilization of the carboxylate anion that forms when a proton is lost.

Synthesis

Carboxyl groups can be introduced into organic molecules by several methods:

• Oxidation of primary alcohols or aldehydes using oxidizing agents such as potassium permanganate (KMnO4) or chromium trioxide (CrO3).
• Hydrolysis of nitriles under acidic or basic conditions to form carboxylic acids.
• Carbonation of Grignard reagents, which involves the reaction of an organomagnesium compound with carbon dioxide (CO2).

Reactions

Carboxyl groups are very reactive and participate in various chemical reactions:

• Formation of esters through reaction with alcohols in a process known as esterification.
• Formation of amides when reacting with amines.
• Reduction to primary alcohols using reducing agents like lithium aluminium hydride (LiAlH4).
• Decarboxylation, which is the loss of the CO2 group, especially under heating conditions.

Biological Importance

Carboxyl groups are pivotal in biology. They are part of amino acids, which are the building blocks of proteins. Carboxylic acids such as Acetic acid and Citric acid play essential roles in metabolism and energy production within cells.

Applications

Carboxylic acids and their derivatives are widely used in industrial applications. They are used in the manufacture of plastics, pharmaceuticals, perfumes, and dyes. They also serve as preservatives and flavoring agents in the food industry.

See Also

• Functional group
• Organic synthesis
• Biochemistry